Solvent system for carboxylic acid manufacture



Dec. 24, 1963 R. BALDWIN ETAL 3,115,520

SOLVENT SYSTEM FOR CARBOXYLIC ACID MANUFACTURE Filed Aug. 12. 1957 WW h OIOV HHZITWLSAHO HHZITIVLSAH United States Patent 3,115,529 SQLVENT Y5TEM PEER CAQXYHC AQED MANUFACTURE Richard H. Baldwin, (Ihicago, 1th, Charles A. Spiller, Jr.,

Maplewood, Nah, and (Iharies D. Kalfadelis, Ham

mend, ind, assignors to ltandard (Bil Company, Chicage, lll., a corporation of Indiana Filed Aug. 12, 1957, Ser. No. 677,595 10 Claims. (Cl. 260-524) This invention relates to an improved solvent system for the manufacture of at least one carboxylic acid product from an aromatic charging stock having at least one and preferably two or more oxidizable substituents. The invention will be described as applied to a system for manufacturing a mixture of phthalic acids from mixed xylenes.

It is known that aromatic hydrocarbons having at least one, and preferably two or more, oxidizable substituents may be converted into carboxylic acid products by contact with an oxidizing gas under carefully controlled conditions in the presence of -a suitable solvent such as a mono-basic aliphatic acid reaction medium having 2 to 6 carbon atoms per molecule such, for example, as acetic acid, and also in the presence of known oxidation catalysts such as manganese and/ or cobalt and also in the presence of a bromine-affording material. The object of this invention is to provide an improved commercial oxidation system of this type which is sufitciently flexible so that it may be employed for oxidizing a wide variety of charging stocks but which is particularly suitable for converting mixed xylenes to mixed dicarboxylic acids in such a form that they may be readily separated into substantially pure phthalic acid (or anhydride), isophthalic acid and terephthalic acid, respectively. A particular object is to provide an improved technique for effecting nucleation and growth of product crystals. A further object is to provide an improved integrated technique for utilizing and recovering solvent medium in and from all parts of the system. An additional object is to provide an improved product drying technique which is integrated with the solvent portion of the oxidation and recovery system. Other objects will be apparent as the detailed description of the invention proceeds.

In practicing the invention the aromatic charging stock having at least one and preferably at least two or more oxidizable substituents may be introduced into either a batch or a continuous multistage oxidation system along with about 1 to 5 volumes of solvent such as a monobasic aliphatic acid reaction medium having 2 to 6 carbon atoms per molecule (preferably acetic acid), an oxidation catalyst and a bromine-affording substance as will be hereinafter described. Oxidation is efiected by introducing an oxygen-containing gas such as air in a carefully controlled manner. While countercurrent oxidation in a single vessel or stage-countercurrent system is preferred, the oxidation may be effected batch-wise; in either case the gasiform stream withdrawn from the oxidizing zone, particularly in the condenser and in the receiver, should not exceed about 8 to 9 percent by volume on a combustible free basis in order to avoid possible explosion hazards.

The oxidation is preferably efiected at a pressure in the range of about 100* to 500 e.g., about 350 p.s.i. at a temperature in the range of about 320 to 440 F., e.g., about 380 F., for a period of time in the range of about .1 to 3 hours or more, a solvent-containing inert gas being withdrawn at high pressure from the oxidizing zone. The product effluent from the oxidizing zone is passed to an initial surge or crystallizing tank, or preferably to additional crystallizing zones, at lower temperatures and pressures and aqueous solvent vapors are removed from 3,115,520 Patented Dec. 24, 1963 these surge and crystallizing zones. The final carboxylic acid product is dried with substantially anhydrous solvent which in turn is removed from the crystals with an inert gas. Solvent-containing inert gas from the oxidizer is scrubbed with Water in the high pressure scrubbing zone while solvent-containing inert gases from other parts of the system are scrubbed with water in the low pressure scrubbing zone. Liquid aqueous solvent streams from these two scrubbing zones and from other parts of the system are charged to a solvent still for removing water from the system and providing the substantially anhydrous solvent which is employed for drying product crystals, providing reaction medium in the oxidizing zone and providing a crystallizing medium in crystallizers.

The particular steps and conditions for obtaining optimum crystallization will be dependent, of course, on the nature of the charging stock and of the desired products. In the case of mixed xylenes obtained from hydroformed naphtha by solvent extraction, it is desirable that the first crystallization stage be at a pressure of about p.s.i.g. (plus or minus 15 to 20 p.s.i.), at a temperature of about 350 F. for crystallized terephthalic acid and nucleating isophthalic acid; that the second crystallizer be operated at about 1 to 2 p.s.i.g. and about 235 F. for crystallizing isophthalic acid and nucleating phthalic acid, and that the final crystallizer be under relatively high vacuum of the order of to millimeters of mercury at a temperature of about F. for crystallizing phthalic acid without nucleating or crystallizing benzoic acid. By operating in this manner, each of the three dicarboxylic acids is separated in crystals of such size and uniformity that substantially all of the mixed acids may be readily separated from mother liquor by centrifuging. The separated acids, after being washed with acetic acid and dried, may subsequently be separated into substantially pure individual dicarboxylic acids by an aqueous separation technique which minimizes corrosion problems. It is important that an integrated system be provided for handling the gasiform and aqueous acid streams that are produced in the various parts of a carboxylic acid production plant.

Further objects and aspects of the invention will be apparent from the following detailed description of a specific example thereof read in conjunction with the accompanying drawing which forms a part of this specification and which is a schematic flowsheet of our improved oxidation, crystallization and purification, and solvent utilization system.

The mixed xylene charge is introduced by line 10 to oxidizing reactor 11 along with solvent introduced through line 12 and catalyst and bromine-affording substance introduced through line 13. Air is introduced at the base of the oxidizer through line 14 in carefully controlled amounts, the total amount being somewhat in excess of that required to supply 3 mols oxygen per mol of xylene charged. The oxidizing vessel 11 may be a stirred stagecountercurrent reaction vessel or any other known type of countercurrent reactor which provides adequate contacting and temperature control. In this example about 1.5 volumes of acetic acid is employed per volume of mixed xylene charge and manganese bromide is employed to supply both the catalyst metal and the bromine, the manganese bromide being added to give about 1.5 weight percent manganese based on xylene charged. The oxidation zone is maintained under a pressure of about 350 to 400 p.s.i.g. at a temperature of about 380 to 400 F. for a time of about an hour.

No novelty is claimed per se in the catalyst or in the catalyst promoter employed in this invention since the invention is an improvement in a particular promoted catalyst system recently perfected by others. The catalyst is a multi-valent or heavy metal, prefer-ably in a form which is soluble in the reaction medium or solvent. The catalyst metal may advantageously be selected from the group consisting of manganese, cobalt, nickel, chromium, vanadium, molybdenum, tungsten, tin, cerium, or mixtures thereof, and the metal may be employed in elemental, combined, or ionic form, preferably in the form of acetate when the reaction medium or solvent is acetic acid. The promoter is a substance-affording bromine, the bromine being in elemental, ionic, organic or inorganic tform. Thus the bromine may be employed in the form of potassium bromate, ammonium bromide, benzyl bromide, tetrabromoethane', manganese bromide or the like. The proportions of catalyst and promoter may be approximately stoiohiometric as exemplified by manganese bromide and generally are in the range of 1:10 to :1 atoms of catalyst metal per atom of bromine. The amount of catalyst employed is usually in the range of about .0 1 to 10 percent or more and is preferably in the range of about .2 to 2 percent by weight based on aromatic hydrocarbon reactant charged. The preferred mixed meta-l catalysts are mixtures of manganese in the form of bromide or acetate with ammonium molybdate, ammonium chromate, tungstic acid, cobalt acetate, or the like, the proportions usually being about 1 to 2 parts by weight of the manganese salt per part by weight of the other metal compound. Alternatively, of course, the catalyst metals may be employed as salts of an acid which is being produced in the system.

The solvent is preferably an aliphatic mono-basic carboxylic acid having about 2 to 6 carbon atoms per molecule such, for example, as acetic acid although it should be understood that other carboxyl-ic acids which, in substantially anhydrous form are liquid under the oxidation and crystallizing conditions, may be suitable.

In order to remove heat of reaction and maintain a substantially uniform temperature in the oxidizing vessel 11, vapors are removed through line 115 and cooler 16 to receiver 17 from which condensate may be withdrawn by pump 18, a part of the condensate being recycled through line 1 9 to the oxidizer and the rest of the condensate being discharged through line 20 and line 21 to solvent still 22. The inert gases vented from the upper part of receiver 17 through line 23 are contacted in high pressure scrubber 24 with water introduced from line 25 so that gases vented through line 26 are substantially free from solvent. The aqueous solvent discharged from the base of the high pressure scrubber through line 27 is also returned by line 21 to solvent still 22.

Whether oxidation is effected in one, two or three vessels and whether it is effected batch-wise or continuously, the oxidized product stream is withdrawn through line 2-8 by pressure reducing valve, pump, or metering means 29 to vessel 30 which may function as a surge tank in a batch oxidizing system or as a crystallizer in a continuous oxidizing system. In this example vessel 30 is maintained at a pressure of about 75 p.s.i.g. and a temperature of about 350 P. so that terephthalic acid crystals will grow to separable size and isophthalic acid will be nucleated. Vapors, mostly aqueous solvent, are withdrawn from vessel 30 by line 31 through cooler 32 to receiver 33 from which uncondensed inert gas and vapors are taken overhead through line 34- and pressure reducing valve 35 to the base of low pressure scrubber 36 which in this case operates at about atmospheric pressure. Water from line 37, scrubbed solvent from vapors which are discharged through line 38 and the aqueous solvent from the low pressure scrubber is returned by line 39 to line 21 and thence to still 22. Liquid from the base of receiver 33 passes by line 40 and pressure reducing valve 41 to line 21.

After about .3 to 3 hours, e.g. 1 hour, holding time in vessel 30, slurry is removed by line 42 and metering or transfer means 43 to vessel 44 which in this case is operated at about 1 to 2 p.s.i.g. and at a temperature of about 235 F. for growing crystals of isophthalic acid to a separable size while nucleating phthalic acid. Vessel 44 may be about the same size or somewhat larger than vessel 36. Aqueous solvent vapors are removed from vessel 44 by line 45 through cooler 4-6 to receiver 47 from wluch uncondensed solvent-containing inert gases are passed by compressor 48 and line 49 to line 34 and thence to low pressure scrubber 36. Condensate from receiver &7 is passed by pump 50 and line 51 to line 21 and thence to still 22.

Slurry from the base of vessel 44- is passed by line 52, by pump, pressure reducing valve or other metering means 53 to vacuum crystallizer 54 which in this case is maintained under a pressure of about 10 0 to 110 millimeters of mercury at a temperature of about 140 F. for obtaining large phthalic acid crystals without nucleating or crystallizing benzoic acid. Inert gas and vapors are removed from vessel 54 through line 55 and cooler 56 to receiver 57 from which solvent-containing gases are passed by vacuum compressor 58 and iline 59 to line 34 and low pressure scrubber 36. Liquid from the base of receiver 57 is pumped by vacuum pump 69 and line 61 to line 21 and thence to receiver 22. The crystallizer 54 may be about the same size as vessel 30 or vessel 44-.

The final slurry from crystallizer 54 is passed by line 62 and pump 63 to separating means 64 which may be a suitable filter or centrifuge system. The charge to the centrifuge may be interrupted by returning slurry pumped by pump 63 back to crystallizer 54 through line 65. Mother liquor is removed from the filter or centrifuge system by line 66; a part of this mother liquor may be recycled to the crystallization system but at least a part thereof is preferably separated by fractional distillation to recover aqueous solvent (to be returned to line 21 and still 22), benzoic acid and any toluic or dicarboxylic acids which may be recycled either to the oxidation zone or to the crystallizing system depending upon their particular composition.

After removal of mother liquor from the crystals, these crystals are washed with substantially anhydrous solvent which is obtained from storage tank 67 via line 6 3 and which after washing the product crystals is introduced by line 69 to wash solvent vessel 76. By washing the product crystals with substantially anhydrous solvent, both water and other impurities are removed from the crystals. The initial part of this wash liquid may be added to mother liquor but the final part of the wash solvent may be directly recycled by line 12 to the oxidizing zone since it will be substantially anhydrous, i.e. will contain less than 5 percent water.

The solvent-washed crystals removed from the separation step are introduced into dryer 72 and countercurrently contacted with inert gas introduced by line 73, a temperature of 200 or 300 F. being maintained in this drying zone. The solvent-enriched inert gas is passed by line 74 and compressor 75 to the base of acid scrubber 76 wherein the upfiowing gasiform stream is scrubbed with cool, substantially anhydrous acetic acid introduced through line 77. A part of the acid stream withdrawn from the base of the scrubber is passed by pump 78 and line 79 to wash solvent tank 70 while another part is recycled by line 80 and cooler 81 for return by line 77 to the top of scrubber 76. Inert gas is withdrawn from the top of scrubber 76 by line 82, a part of this withdrawn gas being recycled by line 73 to dryer 72 as hereinabove described and another part of said gas being introduced by line 83 to line 34 and low pressure scrubber 36.

Dried phthalic acid crystals are withdrawn from the system as product stream 84 and the difierent phthalic acids may be separated from each other in any desired manner.

From the foregoing description it will be observed that solvent-containing gases and solvent-containing liquids are removed in various streams throughout the system. The solvent-containing inert gas vented from the oxidizer is scrubbed with Water in the high pressure scrubber but inert solvent-containing gases from all other parts of the system are introduced at the base of the low pressure scrubber, this particular arrangement having been found to be more economical and efiicient than using a single water scrubber. The use of an acid scrubber in the described drying system enormously simplifies the problem of recovering solvent and entrained solids from the inert gas stream. The solvent recovered from all parts of the system is ultimately introduced by line 21 to still 22 from which substantially anhydrous solvent is returned by line 85 to storage tank 67. The aqueous vapors removed from the top of still 22 by line 86 are cooled in condenser 87 and then introduced into separator 88 from which uncondensed gases may be vented by line 89. Any hydrocarbons entering still 22 will be azeotroped overhead, condensed in cooler 87 and will flow over weir 90 for separate removal through line 91. Condensed Water may be withdrawn by pump 92 to reflux line 93 and discard line 94. While the foregoing example has been described in considerable detail with respect to the oxidation of a particular charging stock, it should be understood that the invention is applicable to a large number of different charging stock and for the production of a large number of polycarboxylic acid products. If a single xylene is employed as a charge instead of mixed xylenes, the crystallization technique may be simplified since the particular system hereinabove described is designed for effecting crystallization of phthalic acid, isophthalic acid and terephthalic acid, in a form which is readily separable from mother liquor and which enables subsequent separation and recovery of these individual dicarboxylic acids in substantially pure form. Examples of oxidizable substituted aromatic hydrocarbons and the acids obtainable therefrom are as follows:

Methylbenzene Benzoic. 1,2-dimethylbenzene Phthalic. 1,3-dimethylbenzene Isophthalic. 1,4-dimethylbenzene Terephthalic. 1,2,3-trimethylbenzene Hemimellitic. 1,2,4-trimethylbenzene Trimellitic. 1,3,S-trimethylbenzene Trimesic. 1,2,3,4-tetramethylbenzene Prehnitic. l,2,4,5-tetramethylbenzene Pyromellitic. 1,2,3,5-tetramethylbenzene Mellophanic. Pentamethylbenzene Benzenepentacarboxylic. Hexamethylbenzene Mellitic. p-diisopropylbenzene Terephthalic. m-diisopropylbenzene Isophthalic. l,3,S-triisopropylbenzene Trimesic. 1,3-dimethyl--ethylbenzene Trimesic. l-methylnaphthalene a-Naphthoic acid. Z-methylnaphthalene fi-Naphthoic acid. 3-methylpyridine Nicotinic acid.

While crystallization is preferably effected by removal of vapors for effecting concentration and/ or temperature control, the crystallization may be effected in a scraped surface exchanger provided that it is constructed of a material which will not be corroded or cause product contamination; the use of a scraped surface exchanger otfers the advantages of close cooling rate control at nucleation temperatures, cooling rate control for obtaining desired crystal sizes and diluent addition as a crystal control expedient, etc. Alternative operating techniques, steps and conditions will be apparent from the foregoing description to those skilled in the art.

We claim:

1. In a process for producing mixtures of isomeric phthalic acids from mixtures of isomeric dialkylbenzenes containing 1 to 3 carbon atoms in the alkyl groups by oxidizing with molecular oxygen in the presence of bromine and a heavy metal oxidation catalyst, said oxidizing being under liquid phase conditions in the presence of a normally liquid monocarboxylic acid rection medium of the acetic acid series having 2 to 6 carbon atoms per molecule, the improved method of operation which comprises introducing said reaction medium containing dissolved phthalic acids from said oxidizing step into a first crystallizing zone operated at superatmospheric pressure, crystallizing from the reaction medium the least soluble phthalic acid in the first crystallizing zone to obtain a first crystal slurry, introducing the first crystal slurry from the first crystallizing zone to a second crystallizing zone operated at approximately atmospheric pressure, crystallizing another phthalic acid in said second crystallizing zone to form a combined crystal slurry, introducing the combined crystal slurry from the second crystallizing zone to a third crystallizing zone operated at subatmospheric pressure, increasing the size of previously formed crystals in said third crystallization zone to form a final crystal slurry and separating said final crystal slurry into a mother liquor and a mixture of separable phthalic acid crystals respectively.

2. The method of claim 1 which includes the step of washing said separable phthalic acid crystals with substantially anhydrous acetic acid to form wash liquid and returning said wash liquid to said oxidation zone for supplying reaction medium thereto.

3. The method of claim 2 which comprises removing reaction medium from Washed crystals with an inert gas and recovering removed reaction medium from the inert gas in a cooled reaction medium scrubber.

4. The method of claim 1 which includes the steps of operating the first crystallizing zone at a pressure of about p.s.i.g. and a temperature of about 350 F, operating the second crystallizing zone at a pressure of about 2 p.s.i.g. and a temperature of about 235 F. and operating the third crystallizing zone at a pressure of about millimeters of mercury at a temperature of about F.

5. The method of claim 1 wherein each, except the last, of the crystallizing zones is operated under conditions for promoting growth of crystals of a particular phthalic acid while nucleating crystals of a more soluble phthalic acid and wherein the last crystallizing zone is operated above the temperature at which crystals of benzoic acid when present would be nucleated While promoting growth of phthalic acid already nucleated.

6. The method of purifying aromatic dicarboxylic acid which is crystallized in acetic acid mother liquor, which method comprises centrifuging said mother liquor from said dicarboxylic acid crystals, washing said dicarboxylic acid crystals with substantially anhydrous acetic acid, removing said substantially anhydrous acetic acid from the crystals by contacting with an inert gas at elevated temperature, recovering acetic acid from inert gas employed in the drying step by scrubbing said inert gas with cooled substantially anhydrous acetic acid liquid in a scrubbing zone, passing a portion of the gas from the top of the scrubbing zone to the elevated temperature contacting step, and recycling a part of the liquid from the base of the scrubbing zone through a cooling zone to the upper part of the scrubbing zone to serve as a scrubbing medium therein.

7. The method of claim 6 which includes the step of Washing the remaining portion of the gas removed from the top of scrubbing zone with water for recovering acetic acid therefrom.

8. In a process for producing polycarboxylic aromatic acids wherein a polyalkyl aromatic compound whose alkyl groups contain 1 to 3 carbon atoms is oxidized in liquid phase to obtain a polycarboxylic aromatic acid in an oxidizing zone in the presence of a monocarboxylic acid solvent of the acetic acid series having 2 to 6 carbon atoms per molecule, a first solvent-containing inert gas stream is discharged from the upper part of said oxidizing zone, a product effluent is discharged from the base of said oxidizing zone and a second solvent-containing gas stream is separated from the product effluent downstream of said oxidizing zone and at a lower pressure, the improved method of operation which comprises scrubbing the first solvent-containing inert gas stream With water in a high pressure scrubbing zone, scrubbing the second solvent-containing inert gas stream with water in a low pressure scrubbing zone, collecting liquid aqueous solvent from both of said scrubbing zones, distilling said aqueous solvent to obtain substantially anhydrous solvent and employing the solvent in the system in substantially anhydrous form.

9. The method of claim 8 which includes the steps of passing product effluent from the oxidizing zone through a superatmospheric pressure crystallizing zone, a substantially atmospheric pressure crystallizing zone and a subatmospheric pressure crystallizing zone, removing, cooling and partially condensing vapors from each of said crystallizing zones, passing uncondensed vapors from each of said crystallizing zones to said low'pressurescrubbing zone and passing condensate from each of said crystallizing zones to said solvent distillation zone; i

10. The method of claim 8 which includes the steps of separating carboxylic acid product from product eflluent,

washing said polycarboxylic acid product with substantially anhydrous acetic acid solvent, removing the solvent from the washed polycarboxylic acid product by contacting this solvent-wet product with an inert gas, recovering at least apart of the substantially anhydrous solvent from said inertgas with cooled substantially anhydrous solventcondensate in a third scrubbing zone and introducing a part of the scrubbed inert gas from the third scrubbingizone to the low pressure scrubber. 

1. IN A PROCESS FOR PRODUCING MIXTURES OF ISOMERIC PHTHALIC ACIDS FROM MIXTURS OF ISOMERIC DIALKYLBENZENES CONTAINING 1 TO 3 CARBON ATOMS IN THE ALKYL GROUPS BY OXIDIZING WITH MOLECULAR OXYGEN IN THE PRESENCE OF BROMINE AND A HEAVY METAL OXIDIZATION CATALYST, SAID OXIDIZING BEING UNDER LIQUID PHASE CONDITIONS IN THE PRESENCE OF A NORMALLY LIQUID MONOCARBOXYLIC ACID RECTION MEDIUM OF THE ACETIC ACID SERIES HAVING 2 TO 6 CARBON ATOMS PER MOLECULE, THE IMPROVED METHOD OF OPERATION WHICH COMPRISES INTRODUCING SAID REACTION MEDIUM CONTAINING DISSOLVED PTHALIC ACIDS FROM SAID OXIDIZING STEP INTO A FIRST CRYSTALLIZING ZONE OPERATED AT SUPERATMOSPHERIC PRESSURE, CRYSTALLIZING FROM THE REACTION MEDIUM THE LEAST SOLUBLE PHTHALIC ACID IN THE FIRST CRYSTALLIZING ZONE TO OBTAIN A FIRST CRYSTAL SLURRY, INTRODUCING THE FIRST CRYSTAL SLURRY FROM THE FIRST CRYSTALLIZING ZONE TO A SECOND CRYSTALLIZING ZONE OPERATED AT APPROXIMATELY ATMOSPHERIC PRESSURE, CRYSTALLIZING ANOTHER PHTHALIC ACID IN SAID SECOND CRYSTALLIZING ZONE TO FORM A COMBINED CRYSTAL SLURRY, INTRODUCING THE COMBINED CRYSTAL SLURRY FROM THE SECOND CRYSTALLIZING ZONE TO A THIRD CRYSTALLIZING ZONE OPERRATED AT SUBATMOSPHERIC PRESSURE, INCREASING THE SIZE OF PREVIOUSLY FORMED CRYSTALS IN SAID THIRD CRYSTALLIZATION ZONE TO FORM A FINAL CRYSTAL SLURRY AND SEPARATING SAID FINAL CRYSTAL SLURRY INTO A MOTHER LIQUOR AND A MIXTURE OF SEPARABLE PHTHALIC ACID CRYSTALS RESPECTIVELY. 